Pagewide inkjet printers employing Memjet® technology are commercially available for a number of different printing formats, including desktop printers, wideformat printers and digital presses. Pagewide printheads typically comprise thousands of microscopic inkjet nozzles, each of which needs to be available for droplet ejection on demand.
Inkjet nozzles must be maintained in a hydrated state in order to function properly. Evaporation of water through the nozzle during uncapped idle periods causes the nozzle to become clogged with a viscous plug of dehydrated ink components (a phenomenon known in the art as “decap”). Dehydrated nozzles blocked with viscous material may be unable to eject a droplet of ink in response to a fire signal. Even if a dehydrated nozzle is still able to eject ink in response to a fire signal, the ejected droplet may be misdirected, have a reduced droplet volume or a reduced ejection velocity, any of which may lead to a reduction in print quality. A period of time in which an uncapped nozzle becomes too dehydrated to eject ink properly is known in the art as the “decap time”.
Inkjet printers usually employ a number of strategies for maintaining healthy nozzles. Typically, this involves a maintenance cycle in which nozzles are wiped and then capped to maintain a humid environment over the nozzles. Periodic ink ejections (“spitting”) may also be used to maintain healthy nozzles. Spitting strategies require a nozzle to eject ink periodically on a timescale shorter than the decap time to avoid a viscous plug forming in the nozzle. Spitting may be performed, for example, during a maintenance cycle, between printing of media sheets or onto print media. For example, U.S. Pat. No. 7,246,876, the contents of which are incorporated herein by reference, describes printing a low-density keep-wet pattern onto a media substrate to ensure that each nozzle of the printhead is fired within a time period less than a decap time of the nozzle.
Notwithstanding some of these strategies for maintaining nozzle health, it would be desirable to increase the decap time of printheads, especially pagewide printheads. One approach to increasing decap times is to formulate inks with certain additives, which inhibit water evaporation from the nozzles. For example, U.S. Pat. No. 8,573,762 describes the use of certain polyurethanes in inks for increasing decap times. However, a problem with incorporating evaporation inhibitors into inks is that the evaporation inhibitor necessarily impacts on the ink properties. Evaporation inhibitors typically have relatively low solubility in inks and may compromise the balance of properties required to formulate an aqueous inkjet ink.
It would therefore be desirable to provide a method of increasing printhead decap times, which does not affect ink formulations.